The AUV’s nosecone will be a free-flooded chamber but needs hold its form for hydrodynamic purposes. I had originally planned to make the nose cone have a spherical form at the front with a radius something smaller than the AUV’s diameter, then blending into the body. The purpose of that was to eventually be able to put a phased-array forward-looking sonar in the nose for mapping and obstacle avoidance, and the spherical form looking forward would reduce the refractive effects of the water-fiberglass-water transition.
However, given the magnitude of the scope of building an AUV and a side looking Synthetic Aperture Sonar, any kind of additional forward-looking sonar would be so far down the road that I opted for a simpler, more hydrodynamic and more aesthetically pleasing elliptical nose-cone form instead.
The first step in making the nose cone was to build a plug, to be used to make a mold. Using 0.75″ blue insulation foam, I cut out profiles using the CNC machine. The profiles were slightly oversized to compensate for any slop in the assembly and cutting, with the excess to be sanded down to the correct dimensions.
The pieces were then glued together, onto a wooden dowel which served to keep everything aligned and to give something for the lathe chuck to bit onto. To keep things tight, I used the mill itself to act as a “clamp”. The joy of working with foam is that you can get away with doing things you shouldn’t do with harder materials…
I cut out another profile, and lined the inside with sandpaper — This was used to help sand the foam to the right shape. Once in the spackle stage, the form was used to help evenly spread the spackle. It would have been better to use a more rigid material for the form, like wood, but I didn’t have any kicking around, so opted to work with what I had.
The first sanding step was to sand the foam past the target dimensions, and then build up a layer of spackling filler – The spackling will help give a smoother, stronger finish than is achievable with foam, which is essential for a good fiberglass mold, and hydrodynamic surface. I used quick-dry spackle to speed up working time, although since it off-gases quite a bit as it dries, I had to use it outside and the cold weather slowed the dry time back down to a crawl. The quick-dry spackle does shrink somewhat as it dries, causing cracking in thick layers, but that’s okay as the cracks are filled in over subsequent, finer applications.
Once a decent layer of spackle was built up over top of the foam, I used the CNC lathe to bring the plug to the correct dimensions and symmetry. To create the G-Code, I used e-Cam running in my Windows VM. Since I didn’t have to rough away all the material from a full cylinder, I created a DXF file with offset profiles from the final shape, starting at 0.25mm and building up to 1mm steps. Importing these, I created a series of “finishing” profiles, which resulted in tracing the final lines.
Normally the Sherline lathe can’t handle very large diameters, but using riser blocks and some custom made tool risers (Another thing I worked on over the break, which I’ll need to finish other AUV parts) I managed to swing the large diameter part. Luckily, the plaster is relatively soft and easy to work with. I’ve run it through the CNC program once, and then added some extra plaster to fill in some gaps where it wasn’t thick enough and will run it through again once that’s dry.
Next steps will be to seal the surface with epoxy resin, polish smooth and wax to perfection — The more time spent working on the plug, the better the end result will be. Since I don’t have a suitable indoor workshop for curing resin (fumes), I’ll need to use a resin formulated to cure at temperatures near freezing so that I can do the epoxy/fiberglass work on the patio. Winter in my neck of the woods is more akin to spring in the rest of Canada and doesn’t go below freezing very often. Unfortunately, this year winter has been a bit colder than usual, so it may be a while before it warms up enough for fiberglass work…
Otherwise, AUV design is slowly progressing. I’ve been taking some more test cuts and refining the CNC programs to generate the universal rings. I’ve been putting more thought into the electronics and control of the AUV, but will leave that as a topic for a future post.